1. Field of the Invention
This invention relates to a method of recovering metal values from spent catalyst and, more specifically, this invention relates to a method of extracting valuable metals from spent desulfurization catalyst by carbothermal reduction.
2. Description of the Prior Art
Desulfurization catalysts utilizing valuable metals as catalytically active components are well-known in the petroleum refining industry. Such catalysts generally comprise a carrier such as aluminum oxide (Al.sub.2 O.sub.3), spinel (MgO.Al.sub.2 O.sub.3) or aluminum silicates, for example, and one or more catalytically active components such as compounds (generally oxides) of cobalt, nickel, molybdenum and tungsten. Such catalysts often contain substantial quantities of Fe.sub.2 O.sub.3 or other iron oxides.
In spite of repeated regeneration, the lifetime of such catalysts is limited due to the accumulation thereon of catalyst poisons, oil residues, carbon, sulfur, moisture and other impurities. Spent catalyst must periodically be replaced by fresh catalyst.
Although spent catalyst contains relatively small amounts of valuable metals, recovery of the metals is important due to the short supply and high cost thereof. Due to environmental considerations, heavy metal-containing catalyst must be disposed of under controlled conditions, often in special dump sites, usually at considerable expense. However, special dumping is not required for substantially metal-free catalyst waste.
Many methods of recovering valuable metals from catalyst wastes are known. These prior known methods each require oxidative roasting of catalyst prior to metal recovery.
One such method is described in Offenlegungsschrift No. 25 56 247 and in U.S. Pat. No. 4,087,510. In this process, spent catalysts are calcined in the presence of alkali salts, preferably soda, whereby metal oxides are converted into water-soluble alkali metal salts. Water-soluble vanadium and molybdenum salts are then obtained by leaching the residue with hot water, but cobalt is generally not recoverable. Satisfactory yields of cobalt can only be obtained if the carrier material, to which cobalt oxide is bound as spinel, is completely dissolved.
Calcining of catalyst in the presence of soda results in the formation of water-soluble alkali aluminates. If SiO.sub.2 contaminants are present, alkali silicates are also formed. Therefore, vary impure metal salt solutions are obtained. Such solutions must be processed further as by precipitation of the aluminates and silicates, leading to considerable losses of molybdenum and vanadium.
Wet chemical methods are known in which spent desulfurization catalyst is treated with aqueous alkaline media without calcining in order to solubilize contaminated alkali molybdate or alkali vanadate salts. For example, in Offenlegungsschrift No. 27 35 684, an aqueous sodium carbonate solution is used. In U.S. Pat. No. 3,567,433, dissolution of molybdenum, vanadium and nickel salts is carried out in a pressurized autoclave in the presence of ammonia or ammonium carbonate. The eluted metal salts may then be subjected to further processing.
In West German Pat. No. 23 16 837, metal salts are dissolved by calcining spent catalyst in the presence of sodium chloride.
In the process of Offenlegungsschrift No. 20 13 945, MoO.sub.3 is sublimed at high temperatures. Following MoO.sub.3 sublimation, the remaining cobalt (which is bound to the residue as spinel) is reduced by roasting and then dissolved with a suitable mineral acid.
All known prior wet chemical methods have the common disadvantage that the highly adsorptive carrier retains substantial amounts of valuable heavy metals, thereby decreasing the yield of the metals and necessitating dumping of residues under special conditions.
In Offenlegungsschrift No. 27 44 573 a method is described in which aluminum oxide corundum, suitable for use as a grinding agent, is prepared from spent catalyst. The alloy obtained with the corundum is contaminated with aluminum, carbon and/or silicon, as well as other materials, depending on the nature of the spent catalyst. The use of such an alloy as a prealloy in the preparation of molybdenum- and cobalt-containing steels is therefore severely limited.
In such a method, sulfur present in the catalyst is transferred to the corundum phase in the form of sulfide, such as aluminum sulfide. Such sulfides are subject to hydrolytic decomposition, resulting in the production of hydrogen sulfide gas and decomposition of the corundum. Consequently, sulfur must be substantially completely removed from the spent catalyst prior to oxidative roasting, or extensive measures must be taken in order to remove the resulting H.sub.2 S and SO.sub.2 from waste gas.
Some known carbothermal reduction processes are carried out at elevated temperatures, e.g. above about 2000.degree. C. However, aluminum oxide melts at about 2000.degree. C. Carbothermal reduction of catalyst at such temperatures necessarily results in reduction of at least a portion of the aluminum oxide, as well as silicon oxides, present in the catalyst. Thus, according to methods using such high temperatures, alloys are obtained with significant amounts of aluminum and silicon, which may interfere in subsequent processing or use of the alloys.